AD8045ARDZ-REEL7(RevB) 데이터 시트보기 (PDF) - Analog Devices
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AD8045ARDZ-REEL7 Datasheet PDF : 26 Pages
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Data Sheet
APPLICATIONS INFORMATION
LOW DISTORTION PINOUT
The AD8045 LFCSP features an Analog Devices new low
distortion pinout. The new pinout provides two advantages
over the traditional pinout. First, improved second harmonic
distortion performance, which is accomplished by the physical
separation of the noninverting input pin and the negative power
supply pin. Second, the simplification of the layout due to the
dedicated feedback pin and easy routing of the gain set resistor
back to the inverting input pin. This allows a compact layout,
which helps to minimize parasitics and increase stability.
The traditional SOIC pinout has been slightly modified as well
to incorporate a dedicated feedback pin. Pin 1, previously a no
connect pin on the amplifier, is now a dedicated feedback pin. The
new pinout reduces parasitics and simplifies the board layout.
Existing applications that use the traditional SOIC pinout can
take full advantage of the outstanding performance offered by
the AD8045. An electrical insulator may be required if the SOIC
rests on the ground plane or other metal trace. This is covered
in more detail in the Exposed Paddle section of this data sheet.
In existing designs, which have Pin 1 tied to ground or to another
potential, simply lift Pin 1 of the AD8045 or remove the potential
on the Pin 1 solder pad. The designer does not need to use the
dedicated feedback pin to provide feedback for the AD8045.
The output pin of the AD8045 can still be used to provide
feedback to the inverting input of the AD8045.
HIGH SPEED ADC DRIVER
When used as an ADC driver, the AD8045 offers results
comparable to transformers in distortion performance.
Many ADC applications require that the analog input signal
be dc-coupled and operate over a wide frequency range. Under
these requirements, operational amplifiers are very effective
interfaces to ADCs. An op amp interface provides the ability to
amplify and level shift the input signal to be compatible with
the input range of the ADC. Unlike transformers, operational
amplifiers can be operated over a wide frequency range down to
and including dc.
Figure 67 shows the AD8045 as a dc-coupled differential driver
for the AD9244, a 14-bit 65 MSPS ADC. The two amplifiers are
configured in noninverting and inverting modes. Both amplifiers
are set with a noise gain of +2 to provide better bandwidth
matching. The inverting amplifier is set for a gain of –1, while
the noninverting is set for a gain of +2. The noninverting input
is divided by 2 in order to normalize its output and make it
equal to the inverting output.
AD8045
This dc-coupled differential driver is best suited for ±5 V
operation in which optimum distortion performance is
required and the input signal is ground referenced.
511Ω
AD8045
511Ω
VCML – VIN
33Ω
VINA
511Ω
VIN
511Ω
511Ω
511Ω
20pF
AD9244
AD8045
VCML + VIN
33Ω
VINB
511Ω
511Ω
0.1µF
2.5kΩ
0.1µF
100Ω
1µF OP27
CML
Figure 67. High Speed ADC Driver
The outputs of the AD8045 devices are centered about the 2.5 V
common-mode range of the AD9244. The common-mode
reference voltage from the AD9244 is buffered and filtered via
the OP27 and fed to the noninverting resistor network used in
the level shifting circuit.
The spurious-free dynamic range (SFDR) performance is shown
in Figure 68. Figure 69 shows a 50 MHz single-tone FFT
performance.
120
100
AD8045
80
60
40
20
0
1
10
100
INPUT FREQUENCY (MHz)
Figure 68. SFDR vs. Frequency
Rev. B | Page 19 of 24
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